Im9

Im9 is the immunity protein made by E. coli at the same time as Colicin E9 to protect itself from the DNase activity of the colicin. Im9 binds with high affinity to colicin E9, and is only released upon entry of colicin E9 to a target E. coli cell. The structure shown is that of the Colicin E9 DNase domain bound to Im9. When Im9 binds to colicin E9, 6 lysine residues become less accessible, and it is hypothesised that that the binding physically blocks one of the major dsDNA binding sites. It is also hypothesised that the conformational dynamics of amino acids in the DNA binding domain change when Im9 binds, again preventing dsDNA binding.

Im9 is more flexible than ColE9, and when the complex is formed the backbone flexibility is then reduced.

3D structures of colicin-E9 immunitiy protein
Colicin Immunity Protein